Well-known build-up processes for producing a multi-layered printed circuit board include an additive process and a semi-additive process in which a conductor pattern is formed on an insulating plate. The above board producing methods use, as the number of layers increases, in order to decrease the processing steps and prevent a noticeable decrease in the yield ratio, a laminating technique of bonding separately produced board layers together with a conducting material or the like in one process.
How a conducting material is supplied and how board layers are stacked up are described with reference to FIG. 6. More particularly, as illustrated in FIG. 6, the conducting material is supplied to a through-hole that is formed on a bonding layer (see (1) of FIG. 6), and then pressure and heat are applied to a plurality of boards, thereby producing a laminated board (see (2) of FIG. 6).
It is noted that an enough amount of the conducting material is supplied to the through-hole of the bonding layer such that, even if the boards that are layers of the laminated board are curved or bumpy, joint lands of the boards are connected to each other. In other words, the conducting material is supplied to the through-hole of the bonding layer with an amount enough even for a position where concave portions of the surfaces of the boards face to each other, and the bonding layer is configured to be thick enough to cancel out any curve and bumpiness on the surfaces of the boards.
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-210766
Patent Document 2: Japanese Laid-open Patent Publication No. 2006-303245
However, since the above conducting-material supplying technique needs to supply a large amount of the conducting material to the through-hole of the bonding layer, it brings a problem in that an excess conducting material flows onto the interface of the bonding layer and this decreases both the isolation performance and the bonding reliability.
Because the bonding layer is thick enough to cancel out any curve and bumpiness on the surfaces of the boards, if there is a position where convex portions of the surfaces of the boards face to each other, the gap between the bonding layers at this position is small. If so, as illustrated in FIG. 7, for example, the excess conducting material flows onto the interface of the bonding layer, which decreases both the isolation performance of the bonding layer and the bonding reliability.